A coherent LIDAR system is an attractive system for determination of wind speed at remote locations. The basic principle relates to the Doppler shift that backscattered light from particles such as aerosols, water droplets, dust, etc. generates when irradiated by coherent laser light. Under the assumption that the particle movements resemble the air-flow closely, the measured Doppler frequency can be used for calculating of a hereto proportional wind speed.
Prohibitive factors for wide spread use of LIDAR are cost, power consumption, size, and ruggedness. Today's commercially available LIDAR systems cost roughly 200 K USD, a price level that exclude many applications to be exploited. The use of LIDAR systems at remote places, such as off-shore wind turbine parks requires operation with little or no maintenance for long periods of time as well as lower power consumption.
Previously the most expensive and complicated component in a coherent LIDAR system was the laser assembly. Due to an extremely low back scatter coefficient of the aerosols (˜10−12) a refined detection scheme is required that relies heavily on high quality, high power, coherent light sources. A low cost, compact and coherent LIDAR system with low power consumption was disclosed in WO 2009/046717 employing an integrated semiconductor laser assembly. This LIDAR system is however limited to line of sight measurements prohibiting the determination of a wind field.
To be able to extent the wind speed measurement beyond the line of sight the probe beam of the LIDAR must be directed in multiple directions. This may be provided by a scanning LIDAR relying on a rotating mirror or prism assembly enabling measurement of the wind velocity in a three dimensional cone. However, having a rotating mirror on top of a wind turbine is not a rugged solution. One or more LIDARs may also be integrated in the rotating hub or on the blades of a wind turbine. Rotation of the hub and the blades ensure that the probe beam scans a cone in front of the wind turbine, but the rotating movements are not really compatible with the operating requirements of LIDAR. WO 03/048804 discloses a LIDAR system employing an optical switch for directing the probe beam in multiple directions. However, the properties of an optical switch require that an expensive laser source with long coherence length is used prohibiting the use of low-cost integrated semiconductor laser assemblies.